Navigation receivers that use the signals of the global navigation satellite systems, such as GPS and GLONASS (hereinafter collectively referred to as “GNSS”) enable highly accurate position determination. A GNSS receiver receives and processes radio signals transmitted by the navigation satellites.
The need to improve positioning accuracies has led to the development of “differential navigation/positioning.” In this mode, the user position is determined relative to the antenna connected to a base receiver (“base”), assuming that the coordinates of the base are known with high accuracy. The base receiver transmits its measurements (or corrections to the full measurements) to a mobile navigation receiver (“rover”). The rover receiver uses these corrections to refine its own measurements in the course of data processing. The rationale for this approach is that since the pseudo-range measurement errors on the base and rover sides are strongly correlated, using differential measurements will substantially improve the positioning accuracy.
Usually, the base is static and located at a known position. However, in relative navigation mode, both the base and rover are moving. In this mode, the user is interested in determining the vector between the base and the rover. In other words, the user is interested in determining the continuously changing position of the rover relative to the continuously changing position of the base. For example, when one aircraft or space vehicle is approaching another for in-flight refueling or docking, a highly accurate determination of the relative position between the two vehicles is important while the absolute position of each vehicle is generally not as critical.
In some situations, a position measurement may not be available because there is a shading of signals, for example. In other words, there may be a disappearance of measurements due to the shading of signals coming from one or several satellites that are tracked by a receiver. As a consequence, in conventional systems, the receiver working in either standalone or differential modes can lose the ability to determine position. To remedy this defect, multiple antennas may be used so that an obstacle does not shade all of the antennas simultaneously.
However, for a single user, portability of a multiple antenna system that is accurate and durable may be difficult. Therefore, a navigation apparatus that is portable and durable enough to preserve the configuration of antennas is desirable. Embodiments of the invention preserve the configuration of the antennas and provide a lightweight and compact assembly that is suitable for transporting. The multi-antenna feature helps to reduce signal shading and to improve antenna accuracy.